Stereochemistry of Alkanes and Cycloalkanes

1. Chapter 4 Stereochemistry of Alkanes and Cycloalkanes

2. Introduction • Stereochemistry • It is the systematic study of the three-dimensional shapes of molecules and properties that arise from these shapes • The three-dimensional shapes of molecules result from many forces

3. Conformations are different shapes that a molecule may assume. • Conformers are conformational isomers. • They are in equilibrium at room temperature. • They can’t usually be isolated because they interconvert too rapidly

4. Alkanes • have C-C single bonds formed by s overlap of sp3 hybrid orbitals • Rotation is possible around s bonds because of their cylindrical symmetry => Many Conformers

5. I. Conformations • Ethane • Propane • Butane

6. Conformers interconvert rapidly and a structure is an average of conformers Representing three dimensional conformers in two dimensions is done with standard types of drawings A.Ethane

7. Molecular models are three dimensional objects that enable us to visualize conformers

8. There are two representations: Sawhorse representation Newman projection Representing Conformations

9. Sawhorse representations show molecules at an angle, showing a molecular model C-C bonds are at an angle to the edge of the page all C-H bonds are shown Newmanprojections show how the C-C bond would project end-on onto the paper Bonds to front carbon are lines going to the center Bonds to rear carbon are lines going to the edge of the circle Representing Conformations

10. Ethane’s Conformations • The most stable conformation of ethane has all six C–H bonds away from each other (staggered). • The least stable conformation has all six C–H bonds as close as possible (eclipsed) in a Newman projection.

11. Ethane’s Conformations • The barrier to rotation between conformations is small (12 kJ/mol; 2.9 kcal/mol) • The eclipsed conformers are 12 kJ/mol higher in energy than the staggered conformers – energy due to torsional strain

12. Ethane’s Conformations • The torsional strain(12 kJ/mol) of the eclipsed conformers are due to 3 H-H eclipsing interactions. • Each H-H interaction contributes 4.0 kJ/mol

13. B.Propane • Propane (C3H8) has torsional barrier around the carbon–carbon bonds (14 kJ/mol). • Eclipsed conformer of propane has two ethane-type H–H interactions and an interaction between C–H and C–C bond

14. The torsional strain(14 kJ/mol) of the eclipsedconformers are due to 2 ethane-type H-H interactions and an interaction between C–H and C–C bond. • The C–H and C–C bond interaction contributes 6.0 kJ/mol (= 14 – (2 x 4.0))

15. Practice Problem: Make a graph of potential energy versus angle of bond rotation for propane, and assign values to the energy maxima

16. Practice Problem: Draw Newman projections of the most stable and least stable conformations of bromoethane

17. C.Butane • As the alkane becomes larger, the conformations become more complex. • Butane has eclipsed and staggered conformers with different energy level around C2-C3:

18. Butane’s Conformations • anti conformation is the most stable conformation of butane • It has two methyl groups 180° away from each other

19. Butane’s Conformations • Rotation around the C2–C3 gives eclipsed conformation

20. Butane’s Conformations • gauche conformation is the staggered conformation with methyl groups 60° apart. • Although it has no eclipsing interactions, it is 3.8 kJ/mol higher in energy than the anti conformation. • This is due to steric strain.

21. Butane’s Conformations • The steric strain(3.8 kJ/mol) of the gauche conformation is due to the repulsive interaction that occurs when atoms are forced together than their atomic radii allow.

22. Butane’s Conformations • The least stable eclipsed conformation is one in which the methyl groups are too close. • 19 kJ/mol is due to steric and torsional strain.

24. For any alkane, the most favorable conformation is: the staggered arrangement on C-C bonds and large substituents arranged anti to one another.

25. One particular conformer is more stable than another means a large percentage of molecules will be found a in more stable conformation than in a less stable one.

26. Practice Problem: Consider 2-methylpropane (isobutane). Sighting along the C2-C1 bond: • Draw a Newman projection of the most stable • conformation • Draw a Newman projection of the least stable • conformation • Make a graph of energy versus angle of rotation around the C2-C1 bond • Since a hydrogen-hydrogen eclipsing interaction costs 4.0 kJ/mol and a hydrogen-methyl eclipsing interaction costs 6.0 kJ/mol, assign relative values to the maxima and minima in your graph

27. Practice Problem: Sight along the C2-C3 bond of 2,3-dimethyl- -butane, and draw a Newman projection of the most stable conformation.

28. Practice Problem: Draw a Newman projection along the C2-C3 bond of the following conformation of 2,3- dimethylbutane, and calculate a total strain energy

29. II. Stability of Cycloalkanes • The Baeyer Strain Theory • Heat of Combustion • The Nature of Ring Strain • Cyclopropane: An Orbital View

30. A.The Baeyer Strain Theory • Baeyer (1885): since carbon prefers to have bond angles of approximately 109°, ring sizes other than five and six may be too strainedto exist.

31. Angle strain is the strain introduced in a molecule when a bond angle deviates from the ideal tetrahedral value, 109°. • Rings from 3 to 30 C’s do exist, despite Baeyer’s theory.

32. B.Heat of Combustion • Heat of Combustion (DH)– is the amount of heat released when the compound burns completely with O2. • The more strain energy, the higher the DH and the less stable the alkane

33. Strain Energy and Heat of Combustion • The higher the n (# CH2), the higher the DH • Therefore, one must compare DH/n rather than DH =n[DH/n cycloalkane -DH/n reference alkane] Strain Energy of Cycloalkane

34. Baeyer’s theory is not fully correct • Cyclopropaneandcyclobutane are strained as predicted. • Cyclopentane is more strained than predicted. • Cyclohexane is strain-free.

35. Practice Problem: Figure 4.8 shows that cyclopropane is more strained than cyclohexane by 115 kJ/mol. Which has the higher heat of combustion on a per-gram basis, cyclopropane or cyclohexane?

36. C.The Nature of Ring Strain • Rings larger than 3 atoms are not flat. • They adopt puckered three-dimensional conformations that allow bond angles to be nearly tetrahedral • Cyclic molecules can assume nonplanarconformations tominimizeangle strain and torsional strain by ring-puckering • Larger rings have many more possible conformations than smaller rings and are more difficult to analyze

37. Cyclopropane has high torsional strain (in addition to angle strain). • This is because C-H bonds on neighboring atoms are eclipsed.

38. Summary: Types of Strain These contribute to the overall energy of a cycloalkane: • Angle strain–is caused by expansion or compression of bond angles away from the normal 109o tetrahedral value • Torsional strain – is caused by eclipsing of bonds on neighboring atoms • Steric strain – is caused by repulsive interactions between nonbonded atoms in close proximity

39. Practice Problem: Each H-H eclipsing interaction in ethane costs about 4.0 kJ/mol. How many such interactions are present interactions are present in cyclopropane? What fraction of the overall 115 kJ/mol (27.5 kcal/mol) strain energy of cyclopropane is due to torsional strain?

40. Practice Problem: cis-1,2-Dimethylcyclopropane has a larger heat of combustion than trans-1,2- dimethylcyclopropane. How can you account for this difference? Which of the two compounds is more stable?

41. D.Cyclopropane: An Orbital View • Cyclopropane was first prepared by reaction of Na with 1,3-dibromopropane:

42. Cyclopropane • 3-membered ring must have planar structure • It is symmetrical with C–C–C bond angles of 60° • All C-H bonds are eclipsed

43. Bent Bonds of Cyclopropane • Cyclopropane requires that sp3based bonds are bent • The orbitals cannot point directly toward each other; they overlap at a slight angle • Cyclopropane bonds are weaker and more reactive

44. Bent Bonds of Cyclopropane • Structural analysis of cyclopropane shows that electron density of C-C bond is displaced outward from internuclear axis

45. III. Conformations of Cycloalkanes • Cyclobutane • Cyclopentane

46. A.Cyclobutane • Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens Cyclopropane (115 kJ/mol strain) Cyclobutane (110.4 kJ/mol strain)

47. Cyclobutane • Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above • The bend increases angle strain but decreases torsional strain

48. B.Cyclopentane • Planar cyclopentane would have no angle strain but very high torsional strain • Actual conformations of cyclopentane are nonplanar,reducing torsional strain • This increases angle strain.

49. Cyclopentane • Four carbon atoms are in a plane • The fifth carbon atom is above or below the plane – looks like an envelope • Most of the H’s are nearly staggered • This increases angle strain but decreases torsional strain

50. Practice Problem: How many H-H eclipsing interactions would be present if cyclopentane were planar? Assuming an energy cost of 4.0 kJ/mol or each eclipsing interaction, how much torsional strain would planar cyclopentane have? How much of this strain is relieved by puckering if the measured total strain of cyclopentane is 26.0 kJ/mol?